Origin of manganese carbonates in Jurassic red shale, central Japan

Manganese carbonate deposits in Japanese Jurassic sedimentary rocks were studied petrogeochemically. The deposits are characteristically composed of spheroidal micronodules, up to 1 mm in diameter, and always contain well-preserved radiolarian shells. Chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition (Mn_86.7?92.2Ca_2.2?2.9Mg_2.6?6.7Fe_2.6?5.6)CO₃ Carbon and oxygen isotope values, which range from ?7.99 to ?4.78‰ and ?4.05 to 0.28‰ relative to PDB, respectively, suggest that the manganese carbonate was precipitated in a suboxic zone. The micronodules closely resemble agglutinated benthic foraminifera in shape. We suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of manganese reduction.     

Celestite formation, bacterial sulphate reduction and carbonate cementation of Eocene reefs and basinal sediments (Igualada, NE Spain)

Petrographic and geochemical studies of an Upper Eocene reef and associated basinal sediments from the mixed carbonate–siliciclastic fill of the south‐eastern Pyrenean foreland basin near Igualada (NE Spain) provide new insights into the evolution of subsurface hydrology during the restriction of a marine basin. The reef deposits are located on delta‐lobe sandstones and prodelta marls, which are overlain by hypersaline carbonates and Upper Eocene evaporites. Authigenic celestite (SrSO₄) is an important component in the observed diagenetic sequences. Celestite is a significant palaeohydrological indicator because its low solubility constrains transportation of Sr²⁺ and SO₄^2? in the same diagenetic fluid. Stable isotopic analyses of carbonates in the reef indicate that meteoric recharge was responsible for aragonite stabilization and calcite cementation. Sulphur and oxygen isotope geochemistry of the celestite demonstrates that it formed from residual sulphate after bacterial sulphate reduction, but also requires that there was a prior episode of sulphate recycling. Meteoric water reaching the reef and basinal areas was most probably charged with SO₄^2? from the dissolution of younger Upper Eocene marine evaporites. This sulphate, combined with organic matter present in the sediments, fuelled bacterial sulphate reduction in the meteoric palaeoaquifer. Strontium for celestite precipitation was partly derived in situ from dissolution of aragonite corals in the reef and basinal counterparts. However, ⁸⁷Sr/⁸⁶Sr data also suggest that Sr²⁺ was partly derived from dissolution of overlying evaporites. Mixing of these two fluids promoted celestite formation. The carbonate stable isotopic data suggest that the local meteoric water was enriched in ¹⁸O compared with that responsible for stabilization of other reefs along the basin margin. Furthermore, meteoric recharge at Igualada post‐dated evaporite deposition in the basin, whereas other parts of the same reef complex were stabilized before evaporite formation. This discrepancy resulted from the spatial distribution of continental siliciclastic units that acted as groundwater conduits.     

Compendium and synthesis of bacterial manganese reduction rates

We have compiled time-series concentration data for the biological reduction of manganese(III/IV) published between 1985 and 2004 and fit these data with a simple hyperbolic rate expression or, when appropriate, one of its limiting forms. The compiled data and rate constants are available in Electronic Annex EA-1. The zero- and first-order rate constants appear to follow a log-normal distribution that could be used, for example, in predictive modeling of Mn-oxide reduction in a reactive transport scenario. We have also included details of the experimental procedures used to generate each time-series data-set in our compilation. These meta-data—mostly pertaining to the type and concentration of micro-organism, electron donor, and electron acceptor—enable us to examine the rate data for trends. We have computed a number of rudimentary, mono-variate statistics on the compiled data with the hope of stimulating both more detailed statistical analyses of the data and new experiments to fill gaps in the existing data-set. We have also analyzed the data with parametric models based on the log-normal distribution and rate equations that are hyperbolic in the concentration of cells and Mn available for reduction. This parametric analysis allows us to provide best estimates of zero- and first-order rate constants both ignoring and accounting for the meta-data.     

The dissolution kinetics of shallow marine carbonates in seawater: A laboratory study

The dissolution kinetics of shallow water marine carbonates (low-Mg calcite, aragonite and Mg-calcites) were investigated in seawater (S = 35) at 25°C and a P_CO2 of 10^?2.5 atm. using the pH-stat method. Carbonate dissoluton rates (μmoles g^?1 hr^?1) fit the empirical kinetic expression, R = k(1 - Ω)ⁿ, where R = dissolution rate, k = rate constant, Ω = saturation state, and n = order of reaction. Reaction orders were near 2.9 for low-Mg calcites, 2.5 for aragonites and 3.4 for Mg-calcites.The rate constant, k, expressed as μmoles g^?1 hr^?1, varied by nearly a factor of ten for the different samples, reflecting differences in amount of reactive surface area. Reactive surface area of the biogenic phases ranged from 0.3% to 66% of the total surface area determined by the BET gas adsorption method. The discrepancy between reactive and total surface area was greatest for samples with high BET surface areas (> 1 m² g^?1) and delicate microstructures.Relative dissolution rates of the various biogenic carbonates as a function of seawater calcium carbonate ion molal product (IMP) were related to both mineral stability and grain microstructure. In seawater undersaturated with respect to aragonite, finely crystalline aragonites dissolved more rapidly than thermodynamically less stable high Mg-calcites (15–18 mole% MgCO₃) with lower reactive surface areas. Therefore, under certain conditions, differences in grain microstructural complexity can override thermodynamic constraints and lead to selective dissolution of a thermodynamically more stable mineral phase.     

The anaerobic degradation of organic matter in Danish coastal sediments: iron reduction, manganese reduction, and sulfate reduction

We used a combination of porewater and solid phase analysis, as well as a series of sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface Mn enrichments reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the less Mn-rich sediments from intermediate depths in the basin, Fe reduction ranged from somewhat less, to far more important than sulfate reduction. Most of the Mn reduction in these sediments may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than to dissimilatory reduction. High rates of metal oxide reduction at all sites were driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These results require that, on average, an atom of Fe or Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment. We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat reduced oxidation level (< 3.8) allowing Mn2+ to escape, and perhaps providing the Mn2+ which enriches sediments of the deep basin.     

Composition of manganese nodules and manganese carbonates from Loch Fyne,Scotland

Manganese nodules and manganese carbonate concretions occur in the upper 10–15 cm of the Recent sediments of Loch Fyne, Argyllshire in water depths of 180–200 m. The nodules are spherical, a few mm to 3 cm in diameter, and consist of a black, Mn-rich core and a thin, red, Fe-rich rim. The carbonate occurs as irregular concretions, 0.5–8 cm in size, and as a cement in irregular nodule and shell fragment aggregates. It partially replaces some nodule material and clastic silicate inclusions, but does not affect aragonitic and calcitic shell fragments.The nodules are approximately 75% pure oxides and contain 30% Mn and 4% Fe. In the cores, the principal mineral phase is todorokite, with a Mn/Fe ratio of 17. The rim consists of X-ray amorphous Fe and Mn oxides with a Mn/Fe ratio of 0.66. The cores are enriched, relative to Al, in K, Ba, Co, Mo, Ni and Sr while the rims contain more P, Ti, As, Pb, Y and Zn.The manganese carbonate has the composition (Mn_47.7 Ca_45.1 Mg_7.2) CO₃. Apart from Cu, all minor elements are excluded from significant substitution in the carbonate lattice.Manganese nodules and carbonates form diagenetically within the Recent sediments of Loch Fyne. This accounts for the high Mn/Fe ratios in the oxide phases and the abundance of manganese carbonate concretions. Mn concentrations in the interstitial waters of sediment cores are high (ca. 10 ppm) as also, by inference, are the dissolved carbonate concentrations.     

Strontium isotope composition of marine carbonates of Middle Triassic to Early Jurassic age,Lombardic Alps,Italy*

The ⁸⁷Sr/⁸⁶Sr ratios and strontium concentrations for thirty-three samples of marine carbonate rocks of Middle Triassic to Early Jurassic age have been determined. The samples were collected from four measured sections in the areas of Val Camonica in northern Italy. The strontium concentrations vary from 40 to 7000 ppm. Most of the samples are calcitic limestones containing less than 10% of non-carbonate residues. Dolomitic samples and those containing appreciable non-carbonate residues have significantly diminished strontium concentrations. ⁸⁷Sr/⁸⁶Sr ratios of the carbonate phases of these rocks appear to be unaffected by dolomitization and by the presence of non-carbonate minerals. The average ⁸⁷Sr/⁸⁶Sr ratios of the formations vary systematically in a stratigraphic sense. The ratio increased from Early Anisian to Early-Middle Ladinian, declined during Late Ladinian and Carnian, rose again during the Norian and then declined throughout the Late Norian (Rhaetian), Hettangian, Sinemurian and Pliens-bachian ages. The average ⁸⁷Sr/⁸⁶Sr ratios, relative to 0.7080 for the Eimer and Amend standard, are: Anisian: 0.70805 ± 00019; Early Ladinian: 0.7085 ± 0.00038; Late Ladinian: 0.70791 ± 0.00013; Carnian: 0.70776 ± 0.00015; Norian and Rhaetian: 0.70791 ± 0.00014; Hettangian: 0.70762 ± 0.00021; Sinemurian: 0.7070 ± 0.00038; Pliensbachian: 0.7070 ± 0.00015. These variations reflect changes in the isotopic composition of Sr entering the oceans in early Mesozoic time due to varying rates of weathering and erosion of young volcanic rocks (low ⁸⁷Sr/⁸⁶Sr) and old granitic rocks (high ⁸⁷Sr/⁸⁶Sr). The data presented in this report contribute to a growing body of information regarding the changes that have occurred in the ⁸⁷Sr/⁸⁶Sr ratio of the oceans in Phanerozoic time.     

Carbon and oxygen isotopes of Maastrichtian–Danian shallow marine carbonates: Yacoraite Formation, northwestern Argentina

The Maastrichtian–Danian limestones of the Yacoraite Formation (northwestern Argentina) show carbon and oxygen isotopic values consistent with shallow marine conditions. The members of the formation respond to different sedimentary environments and are characterised by distinctive stable isotopes and geochemistry. The basal Amblayo Member is composed of high-energy dolomitic limestones and limestones with positive isotopic values (+2‰ δ¹³C, +2‰ δ¹⁸O). The top of the member reveals an isotopic shift of δ¹³C (−5‰) and δ¹⁸O (−10‰), probably related to a descent in the sea level. The sandy Güemes Member has isotopically negative (−2‰ δ¹³C, −1‰ δ¹⁸O) limestones, principally controlled by water mixing, decreased organic productivity, and compositional changes in the carbonates. The isotopically lighter limestones are calcitic, with a greater terrigenous contribution and different geochemical composition (high Si–Mn–Fe–Na, low Ca–Mg–Sr). These isotopic and lithological changes relate to the Cretaceous–Palaeogene transition. The Alemanía Member, composed of dolomitic limestones and pelites, represents a return to marine conditions and shows a gradual increase in isotopic values, reaching values similar to those of the Amblayo Member. The Juramento Member, composed of stromatolite limestones, shows isotopic variations that can be correlated with the two well-defined, shallowing-upward sequences of the member.     

Strontium and carbon isotope stratigraphy of the Late Jurassic shallow marine limestone in western Palaeo-Pacific,northwest Borneo

Strontium and carbon isotope stratigraphy was applied to a 202 m-thick shallow marine carbonate section within the Late Jurassic Bau Limestone at the SSF quarry in northwest Borneo, Malaysia, which was deposited in the western Palaeo-Pacific. Strontium isotopic ratios of rudist specimens suggest that the SSF section was formed between the latest Oxfordian (155.95 Ma) and the Late Kimmeridgian (152.70 Ma), which is consistent with previous biostratigraphy. The δ¹³C_carb values of bulk carbonate range from −0.10 to +2.28‰ and generally show an increasing upward trend in the lower part of the section and a decreasing upward trend in the upper part of the section. A comparable pattern is preserved in the δ¹³C_org isotope record. Limestone samples of the SSF section mainly preserve the initial δ¹³C_carb values, except for the interval 84–92 m, where an apparent negative anomaly likely developed as a result of meteoric diagenesis. Comparing with the Tethyan δ¹³C_carb profile, a negative anomaly in the lower SSF section can be correlated with the lowered δ¹³C values around the Oxfordian/Kimmeridgian boundary. In addition, δ¹³C_carb values of the Bau Limestone are generally ∼1‰ lower than the Tethyan values, but comparable with the values reported from Scotland and Russia, located in Boreal realm during the Late Jurassic. This suggests that either the Tethyan record or the other records have been affected by the δ¹³C values of regionally variable dissolved inorganic carbon (DIC). The Late Jurassic δ¹³C_DIC values are thought to have been regionally variable as a result of their palaeoceanographic settings. This study shows that δ¹³C chemostratigraphy of the Palaeo-Pacific region contributes to an improved understanding of global carbon cycling and oceanography during this time period.     

Burial-dominated cementation in non-tropical carbonates of the Oligocene Te Kuiti Group, New Zealand

Cementation of bryozoan-echinoid-benthic foraminiferal temperate shelf carbonates of the Oligocene Te Kuiti Group, North Island, New Zealand, occurred mainly during subsurface burial. The calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan (given an iron supply) and, compared to the skeletal material with normal marine δ¹⁸O values from +2 to −1‰, more depleted in ¹⁸O with depth of burial, the δ¹⁸O composition of bulk cement samples ranging from −1 to −7‰. These trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures (20–50°C), respectively. The δ¹³C values (0 to +3‰) of the cements remain the same as the host marine shells, suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution.

Two gradational burial diagenetic environments influenced by marine-derived porewaters are arbitrarily distinguished: shallow burial phase and moderate burial phase. During the shallow burial phase, down to 500–600 m sub-bottom depth, the carbonates lost at least 25% of their original porosity by mechanical compaction and were selectively cemented by non-ferroan or usually ferroan, variably luminescent, slightly ¹⁸O-depleted sparry calcite cement (δ¹⁸O −2 to −4‰), mainly as syntaxial rims about echinoid grains. These shallow-burial cements form less than about 10% of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction. During the moderate burial phase, between about 600 and 1100 m sub-bottom depth, porosity loss continued (typically to about 70% of its original value) as a result of pressure-solution of calcitic bioclasts associated with more advanced stages of chemical compaction. This involved development of a wide variety of non-sutured and microstylolitic solution seams, including both single and composite, wispy or continuous, bedding-parallel types and non-parallel reticulate forms. The released carbonate was precipitated as ferroan (or non-ferroan where iron supply was negligible), dull luminescent, strongly ¹⁸O-depleted (δ¹⁸O −4 to −7‰), mainly equant calcite spar cement, occluding available pore space in the limestones.     

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸O values in most concretion bodies range from ?10·0‰ to ?17·3‰ and +0·3‰ to ?0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.     

淮北平原浅层地下水中铁和锰的空间差异性及影响因素

通过常规和地统计学分析,查明淮北平原浅层地下水中铁锰含量的空间差异性,得出变异函数图符合指数模型,二者都由结构性因素引起。对其空间分布特征进行分析表明,铁、锰在空间呈现相同的分布趋势,超标区主要分布在地势低洼地和低山丘陵区,其含量与岩土成分、地下水所处的氧化还原环境、地下水径流条件、酸碱性等有关。     

川东北下侏罗统大安寨段陆相页岩方解石成因

为分析陆相页岩中方解石的成因及其对储层的影响,以四川盆地下侏罗统大安寨段的介壳泥岩与灰岩夹层为主要研究对象,利用X-射线衍射、普通薄片、岩芯观察、阴极发光及电子探针测试手段,识别出文石转化方解石、胶结物方解石和重结晶作用形成的方解石.在同生期和成岩早期,生物介壳中的文石转化为泥晶无铁方解石,阴极发光为橙黄色.成岩早期,...     

Calcium-isotope fractionation in selected modern and ancient marine carbonates

The calcium-isotope composition (δ^44/42Ca) was analyzed in modern, Cretaceous and Carboniferous marine skeletal carbonates as well as in bioclasts, non-skeletal components, and diagenetic cements of Cretaceous and Carboniferous limestones. In order to gain insight in Ca²⁺_aq-CaCO₃-isotope fractionation mechanisms in marine carbonates, splits of samples were analyzed for Sr, Mg, Fe, and Mn concentrations and for their oxygen and carbon isotopic composition. Biological carbonates generally have lower δ^44/42Ca values than inorganic marine cements, and there appears to be no fractionation between seawater and marine inorganic calcite. A kinetic isotope effect related to precipitation rate is considered to control the overall discrimination against ⁴⁴Ca in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the δ^44/42Ca values with Sr concentrations in Cretaceous limestones that contain biological carbonates at various stages of marine diagenetic alteration. No significant temperature dependence of Ca-isotope fractionation was found in shells of Cretaceous rudist bivalves that have recorded large seasonal temperature variations as derived from δ¹⁸O values and Mg concentrations. The reconstruction of secular variations in the δ^44/42Ca value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modern and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the δ^44/42Ca values of Cretaceous skeletal calcite suggest that the δ^44/42Ca value of Cretaceous seawater was 0.3-0.4‰ lower than that of the modern ocean.     

The dependence of bacterial sulfate reduction on sulfate concentration in marine sediments

The effect of dissolved sulfate concentration on the rate of bacterial sulfate reduction in marine sediment from Long Island Sound was examined using a radio-sulfur technique. The experimental results show that the rate is independent of the dissolved sulfate concentration until low levels are reached (<3 mM), and that, when interpreted using a Monod-type rate law, a saturation constant, K_s, of 1.62 ± 0.16 M results. This weak dependence implies that the dissolved sulfate exerts only a limited influence on the rate of sulfate reduction in marine sediments. Given such a weak dependence, dissolved sulfate profiles in marine sediments must resemble profiles generated by models with sulfate independent kinetics. Initially, this would suggest that currently used sulfate-independent diagenetic models are appropriate in modelling sulfate profiles. However, comparison of these models with those containing weak sulfate-dependent kinetic terms shows that there exists considerable disagreement between these models when the parameter grouping $\text{(D}{}_{\mathrm{s}}\text{k)}\text{1}\text{2}\text{/w}$ is larger than ~0.2 and smaller than ~3.0. (Here D_s is the SO^;₄ diffusion coefficient, k the organic matter decay constant and w the sediment burial velocity.) When the currently used models are corrected by employing physically meaningful boundary conditions, this divergence disappears. The modelling results, therefore, confirm the conclusion that any sulfate dependence inherent to the reduction kinetics does not appreciably affect sulfate pore water profiles, and that previous diagenetic studies using strong sulfate dependent models are erroneous.     

Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean

Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ¹⁸Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ¹³C values together with ⁸⁷Sr/⁸⁶Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ¹⁸Ο and δ¹³C values and ⁸⁷Sr/⁸⁶Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ¹⁸Ο, δ¹³C and ⁸⁷Sr/⁸⁶Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.     

Organotemplate structures in sedimentary manganese carbonates of the Neoproterozoic Penganga Group, Adilabad, India

Manganese carbonates interstratified with bedded chert in the Chanda Limestone of the Neoproterozoic Penganga Group at Adilabad, south India, have been studied for possible evidence that microbiota played a role in the mediation of early diagenetic Mn-carbonate formation in Precambrian marine sedimentary successions. The manganese carbonate and chert beds occur within a below wave base, deep-water distally steepened ramp succession. High resolution SEM petrography of the manganese carbonates revealed two basic morphologies-spherical to oval-cylindrical shaped microconcretions, and tubular to irregular, elongated, film-like microstructures. Infolded filmy to hollow tubular strand-like internal morphologies of the spherical to oval-cylindrical shaped microconcretions suggest their microbial affinity. The tubular and film morphologies with mesh-like interconnections closely resemble architectures of microbial extracellular polymeric substance (EPS). Mineralization took place on these organotemplates by the process of permineralization as well as replacement in an early diagenetic pore-water environment with reduction of higher manganese oxy-hydroxides by organic matter and consequent increase in dissolved carbonate.     

Particulate iron and manganese in the Santa Barbara Basin,California

Particulate Fe and Mn may be important trace metal scavengers in the water column as well as being probable indicators of biologically mediated redox processes. A study has been made of suspended particulate composition in the Santa Barbara Basin, a shallow near-shore basin off southern California with sub-oxic conditions below sill depth. Observations have revealed several interesting phenomena relating to the geochemistry of Fe and Mn. Most striking is a profound enrichment of particulate Fe in samples from the bottom two hundred meters. These particulates have a constant Fe/P mole ratio of about three and may originate at the sediment-water interface or may be transported to the basin from local marshes. For particulate Mn, enrichments are observed both in the sub-sill waters and near the base of the euphotic zone. A consideration of particle removal rates suggests that the sub-photic zone enrichment has a biogenic origin. In the sub-sill waters, enrichment in Mn is apparently due to the precipitation of dissolved Mn diffusing from the anoxic basin sediments. A simple mass balance suggests that most of the Mn lost from the sediments is transported from the Santa Barbara Basin in dissolved form.     

Sulfur isotope fractionation during bacterial sulfate reduction in organic-rich sediments

Isotope fractionation during sulfate reduction by natural populations of sulfate-reducing bacteria was investigated in the cyanobacterial microbial mats of Solar Lake, Sinai and the sediments of Logten Lagoon sulfuretum, Denmark. Fractionation was measured at different sediment depths, sulfate concentrations, and incubation temperatures. Rates of sulfate reduction varied between 0.1 and 37 micromoles cm-3 d-1, with the highest rates among the highest ever reported from natural sediments. The depletion of 34S during dissimilatory sulfate reduction ranged from 16% to 42%, with the largest 34S-depletions associated with the lowest rates of sulfate reduction and the lowest 34S-depletions with the highest rates. However, at high sulfate reduction rates (>10 micromoles cm-3 d-1) the lowest fractionation was 20% independent of the rates. Overall, there was a similarity between the fractionation obtained by the natural populations of sulfate reducers and previous measurements from pure cultures. This was somewhat surprising given the extremely high rates of sulfate reduction in the experiments. Our results are explained if we conclude that the fractionation was mainly controlled by the specific rate of sulfate reduction (mass cell-1 time-1) and not by the absolute rate (mass volume-1 time-1). Sedimentary sulfides (mainly FeS2) were on average 40% depleted in 34S compared to seawater sulfate. This amount of depletion was more than could be explained by the isotopic fractionations that we measured during bacterial sulfate reduction. Therefore, additional processes contributing to the fractionation of sulfur isotopes in the sediments are indicated. From both Solar Lake and Logten Lagoon we were able to enrich cultures of elemental sulfur-disproportionating bacteria. We suggest that isotope fractionation accompanying elemental sulfur disproportionation contributes to the 34S depletion of sedimentary sulfides at our study sites.     

Carbon and oxygen isotope ratios in diagenetic carbonates from marine sediments

The isotopic composition of some sixty samples of penecontemporaneous diagenetic carbonates from marine horizons shows that they are strikingly different from normal marine limestones. The variation of C¹³ ratio is 5·6 per cent, the lightest carbonate measuring -54%. A mechanism for the formation of such light carbon dioxide is postulated, involving the enzymic decarboxylation of organic compounds at low temperatures. A study of the oxygen isotopic composition of coexisting calcite and rhodochrosite indicates that, whereas the latter has retained its isotopic composition from the time of precipitation, the calcite has undergone equilibration during diagenesis. Isotopic measurements on such diagenetic carbonates would confuse palaeoenvironmental studies but may throw light on the processes of diagenesis.